1. Field of the Invention
This invention relates to small propulsion systems for maneuvering spacecraft and, more particularly, to an electrothermal arcjet thruster having an anode with a biangle expansion portion and contoured internal regeneration channels to more efficiently convert thermal energy of a flowing propellant to kinetic energy.
2. Description of the Prior Art
An electrothermal arcjet thruster converts electrical energy to thermal energy by heat transfer from an arc discharge to a flowing propellant. The thermal energy is converted to directed kinetic energy by expansion of the heated propellant through a nozzle.
Most electrothermal arcjet thrusters have as common features an anode in the form of a nozzle body and a cathode in the form of a cylindrical rod with a conical tip. The nozzle body has an arc chamber defined by a constrictor in a rearward portion of the body and a nozzle in a forward portion thereof. The cathode rod is aligned on the longitudinal axis of the nozzle body with its conical tip extending into the upstream end of the arc chamber in spaced relation to the constrictor so as to define a gap therebetween.
When a sufficiently high current is applied, an electric arc is initiated between the cathode rod and the anode nozzle body at the entrance to the constrictor. The arc is then forced downstream through the constrictor by pressurized vortex-like flow of a propellant gas introduced into the arc chamber about the cathode rod. The arc stabilizes and attaches at the nozzle. The propellant gas is heated in the regions of the constrictor and of arc diffusion at the mouth of the nozzle downstream from the constrictor. The super-heated gas is exhausted out from the nozzle to achieve thrust.
Historically, propellants, such as ammonia or hydrogen, have been used in electrothermal arcjet thrusters. More recently, hydrazine (N.sub.2 H.sub.4) has been used. Propellants such as ammonia and hydrazine are preferred because these propellants are storable as a liquid without refrigeration while cryogenic propellants such as hydrogen and helium are not. The liquid storable fuels are converted to a gaseous propellent by passing the fuel through a gas generator.
The specific impulse (I.sub.sp) determines the propellant mass required to complete a flight. I.sub.sp is denoted in pounds of force-second per pound of mass. The generation of a high I.sub.sp in an arcjet thruster requires operation of the thruster at a high specific energy (as denoted in watts/kg). The cryogenic propellants have a typical I.sub.sp value of up to 1,500 lbf-sec/lbm. The liquid storable propellants have a much lower specific impulse, on the order of 800-1000 lbf-sec/1 bm.
One way to increase I.sub.sp is to increase the thrust efficiency of the arcjet thruster. U.S. Pat. No. 5,111,656 to Simon et al, discloses increasing the specific impulse of a propellant by a unique nozzle configuration. The exhaust portion of the nozzle has a divergent recombination portion in tandem with a divergent expansion portion. The divergence of the recombination portion is less than that of the expansion portion, causing a temporary delay in the pressure reduction of the propellant gas. This delay creates a relatively high pressure region in the recombination portion of the nozzle permitting a partial recombination of the ionized and neutral species of the propellant gas and a partial recovery of frozen flow losses back into the gas.
Frozen flow losses reduce the efficiency of an arcjet thruster. Frozen flow losses include ionization, disassociation and deposition of energy into excited molecular and atomic states. These losses occur when the propellant gas is heated to very high temperatures by close contact with the electric arc and is then exhausted out the nozzle. If the propellant dwells for insufficient time in high pressure regions, the gas does not have time to recombine the ions or disassociated molecules or to relax the excited states. Energy locked into these processes is lost and unavailable for thrust.
U.S. Pat. No. 5,111,656 is incorporated by reference in its entirety herein. The biangle nozzle disclosed in that patent increases the efficiency of the electrothermal arcjet thruster at low power levels by reducing frozen flow losses. However, the nozzle also generates more heat at the anode surface and, as the energy level (power/mass flow rate) of the thruster increases, the advantage over a single angle nozzle decreases. At relatively high specific energy levels, the efficiency of a biangle nozzle is inferior to that of a single angle nozzle.
The biangle nozzle converts more chemical energy to thermal energy than a single angle nozzle. When operating at high specific energy, most of this extra energy in the form of heat is lost by conduction into the anode or exhausted out the nozzle and does not assist in improving the efficiency of the electrothermal arcjet thruster.
It is known to preheat a propellant gas by flowing the gas through regeneration channels in the anode body prior to exposure to the electric arc. The anode body is heated by the plasma arc and by the disassociating propellant gas. A portion of this heat is recaptured by the propellant gas flowing through the regeneration channels. Preheating the propellant gas forms a more reactive form of propellant gas as disclosed in both U.S. Pat. Nos. 4,548,033 to Cann and 4,995,231 to Smith et al, both of which are incorporated by reference in their entireties herein.
The regeneration channels may assist in cooling the anode, reducing thermal stress on the thruster materials and extending thruster life. However, since the propellant gas has a lower coefficient thermal conductivity than the anode body, the regeneration chambers may also constitute a thermal insulator.
There exists, therefore, a need for a biangle nozzle for an electrothermal arcjet thruster that provides increased thrust efficiency at relatively high specific power levels.